Cobalt metatitanate-modified solid solution

ABSTRACT

A COHERENT MIXTURE OF CRYSTALLINE COBALT METATITANATE WITH A VALVE METAL OXIDE-PLATINUM METAL OXIDE SOLID SOLUTION IS PROVIDED. TYPICAL USE OF THE MIXTURE IS AS AN ADHERENT ELECTRICALLY-CONDUCTIVE ELECTROCATALYTICALLY-ACTIVE COATING APPLIED TO AN ELECTRICALLY-CONDUCTIVE SUBSTRATE FOR USE AS AN ELECTRODE.

United States Patent 3,778,363 COBALT METATlTANATE-MODIFIED SOLID SOLUTION James M. Kolb, Mentor, and Kevin J. OLeary, Cleveland Heights, Ohio, assignors to Electronor Corporation, Panama City, Panama No Drawing. Continuation-impart of abandoned application Ser. No. 104,743, Jan. 7, 1971. This application Feb. 2, 1972, Ser. No. 222,995

Int. Cl. B01k 3/06 US. Cl. 204-290 F '16 Claims ABSTRACT OF THE DISCLOSURE A coherent mixture of crystalline cobalt metatitanate with a valve metal oxide-platinum metal oxide solid solution is provided. Typical use of the mixture is as :an adherent electrically-conductive electrocatalytically-active coating applied to an electrically-conductive substrate for use as an electrode.

REFERENCE TO CO-PENDING APPLICATION This is a continuation-in-part of our co-pending Ser. No. 104,743, filed Jan. 7, 1971, now abandoned.

BACKGROUND OF THE INVENTION In the search for a satisfactory dimensionally stable electrode for use in a number of commercial electrolytic processes an electrode has recently been developed which has met with considerable commercial success. This electrode consists in general of a valve metal substrate bearing on its surface a valve metal oxide-platinum metal oxide solid solution-type coating. In such coatings atoms of platinum metal are randomly substituted for atoms of valve metal in the characteristic rutile valve metal oxide crystal lattice. Such electrodes exhibit remarkably low potentials when employed in a variety of electrolytic processes, e.g., as anodes for the production of chlorine by electrolysis of sodium chloride solutions. A further advantage of such electrodes is that they exhibit a relatively low wear-rate, that is, only a small amount of platinum metal is consumed per ton of product manufactured.

Such electrodes are not entirely without disadvantage, however, in that owing to the amount of platinum metal required to be incorporated in the coating, the electrodes are expensive to fabricate. In addition, while the wearrate is low, generally, for example, on the order of 0.10- 0.15 gram of platinum metal per ton of chlorine produced, considering the tonnages involved a significant amount of platinum metal is irretrievably lost. Further, once the platinum metal content of these solid solutions coatings is substantially depleted, the electrodes become inactive. Hence, the cell must be disassembled for removal and replacement of the electrodes. For these reasons, the search for a coating material exhibiting all the advantages of the solid solution-type coating, with the further advantage of a reduced wear-rate, continues.

STATEMENT OF THE INVENTION A composition has now been found which is especially useful for application as an electrode coating, which composition consists essentially of a coherent mixture of crysice talline cobalt metatitanate with a valve metal oxide-platinum metal oxide solid solution. When applied to a supporting substrate, especially an electrically-conductive supporting substrate and particularly a valve metal substrate, an electrode is obtained which exhibits a low potential and which has extremely low platinum metal wear-rates.

It has further been found that such an electrode is particularly effective if applied in a manner which allows rapid initial heating of the coating during formation and subsequent post-treatment at an elevated temperature. In this manner the physical form of the cobalt metatitanate is apparently optimized, resulting in a particularly durable coating.

The invention finds particular advantage when the coherent mixture of crystalline cobalt metatitanate with a valve metal oxide-platinum metal oxide solid solution is applied to a valve metal base, the resulting structure being used as an anode for the production of chlorine by electrolysis of brine. In this manner a reduction in the amount of platinum metal required per square foot of anode surface is possible, without sacrifice in the potential at which chlorine is discharged at the anode face. Surprisingly, the anode has a significantly lower platinum metal wear-rate per ton of chlorine produced.

DESCRIPTION OF 'I'I-IS PREFERRED EMBODIMENTS A sample electrode has been prepared as is described more fully hereinbelow by application to a clean titanium substrate of a solution containing salts of titanium, ruthenium and cobalt. The electrode exhibits remarkable adhesion between the coating and the substrate. Examination by X-ray diffraction indicates, in addition to the presence of the characteristic rutile TiO RuO solid solution crystal structure, that there is also present crystalline cobalt metatitanate as a separate phase dispersed throughout the solid solution. It is apparent from the results obtained that the cobalt metatitanate in some manner stabilizes the solid solution, thus preventing normal wear to a great extent. To qualify as crystalline for the purpose of this invention the metatitanate, when subjected to X may diffraction analysis, must exhibit peaks characteristic of cobalt metatitanate. It is not necessary that the crystals be highly ordered, although this is preferred, only sufiiciently so that discernable peaks are evidenced. While cobalt metatitanate having at least some degree of crystallinity is required and constitutes the invention in admixure with the solid solution, cobalt oxides and/or amorphous cobalt titanate may be present without substantial detriment. As is explained more fully hereinbelow, the extent of cobalt titanate crystallinity appears to depend upon the various heat treatments to which the electrode is subjected during preparation.

The solid solutions contemplated by the present invention have been described as valve metal oxide-platinum metal oxide solid solutions. By valve metal it is intended to refer to titanium, tantalum, zirconium'and niobium, while by platinum metal the reference is to platinum, palladium, ruthenium, iridium, rhodium and osmium.

The quantities of the various ingredients of the composition of the present invention are conveniently expressed in terms of the mole ratios of the various metals present. Thus the ratio of valve metal to platinum metal plus cobalt should be within the range of 5:1-1z5 while the ratio of platinum metal to cobalt should be within the range of 4:1-2z3. A preferred range, especially when' Generally the use of the compositions of the present invention will be as a coating applied to a conductive substrate. However, the material itself, absent a substrate, will find some application. Techniques for preparation are in many respect standard to the preparation of solid solutions, with the additional inclusion of a cobalt compound. For example, sealed tube or flame spray techniques may be applied. The material thus obtained has a high degree of non-selective catalytic activity thus suggesting its use for the catalytic destruction of organic compounds, such as in exhaust emission. Alternately the material may be impregnated into porous graphite for use as a fuel cell electrode or dispersed in an organic binder, such as a fluorocarbon binder, to prepare a porous electrode, again useful in fuel cells. Other applications will suggest themselves to those skilled in the art.

Generally, however, the composition finds application as a coating on a supporting substrate. Since the coating will not generally be sufiiciently massive to conduct current from the primary source, an electrically-conductive substrate will be employed. Additionally, the substrate will be one which, upon the possible mechanical failure of the applied coating, will be inert to the environment in which it is employed. Preferably, at least when use is to be as an anode for the production of chlorine, the substrate will be a valve metal, typically titanium. The physical configuration of the supporting substrate is independent of the present invention and depends primarily upon the cell design in which the electrode is to be employed. It is also quite possible to apply the coating to a substrate to which there has previously been applied a protective or barrier layer to achieve a desired chemical or physical effect, for example, a manganese dioxide layer serving to prevent the dilfusion of oxygen through the coating to the underlying substrate. Alternately, protective ceramic layers may be applied over the coating, as is known in the art.

While the invention is not so limited, the preferred method of applying the coating composition of the present invention to a supporting substrate, particularly a valve metal substrate, involves the thermochemical deposition of the coating in a number of successive layers. Generally in such a technique, salts of the various component metals are dissolved, for example in alcohol, and applied, as by painting onto the substrate. The solutioncoated substrate is then heated to an elevated temperature in the presence of oxygen to thermochemically convert the salts into the desired chemical and physical form. Successive layers, e.g., 4-12, are applied in this manner until the desired coating thickness is obtained.

While the foregoing general technique has been used with success in the preparation of the usual unmodified solid solution-type coatings, it has been found that certain alterations in the procedure are required if crystalline cobalt metatitanate is to be obtained. Ordinarily the substrate with the applied solution is introduced relatively gradually into an oven and brought to the temperature required for thermochemical conversion. It has been found that to prepare a crystalline cobalt metatitanatemodified coating, however, it is necessary to raise the temperature of the substrate rapidly to within the desired final range. In this manner a larger proportion of cobalt metatitanate in the desired crystalline structure, as opposed to oxides or amorphous titanates, is formed. In addition, it has been found that, after application of the last solution coating and conversion to the desired chem-.

ical form, the coated substrate is preferably subjected to a post-bake treatment at an elevated temperature which treatment further orders the crystals. It has been noted that electrodes subjected to this post-bake exhibit improved wear-rates. It is thought to be surprising that crystalline cobalt metatitanate results from this procedure since known means for production of same generally require temperatures within the range g 800-1000 C.

Thus, for example, a preferred method of preparing an electrode comprises:

(a) applying a solution of salts of titanium ruthenium and cobalt to an electrically-conductive substrate;

(b) rapidly inserting the substrate with applied solution into an oven at a temperature within the range of 425- 475 C. and in the presence of oxygen;

(c) maintaining the substrate at the oven temperature for a period of time sufficient to thermochemically convert the salts to the desired chemical state;

(d) cooling the substrate;

(e) applying any desired number of coatings of said solution in the same manner and,

(f) after thermochemical conversion of the last solution application, heating the substrate to a temperature of 525 575 C. for from 5 to 10 minutes.

In order that those skilled in the art may more readily understand the present invention, the following specific examples are afforded. In these examples, wear-rates are determined by applying the coatings to a six inch square expanded titanium substrate and employing sam: as the anode in a horizontal mercury cell. The anode-cathode gap is established at 0.15 inch, the current density at 6 amperes per square inch, the temperature of the brine (290 grams per liter aqueous sodium chloride) at 160 F., the brine flow rate at 425 milliliters per minute and the mercury flow rate at 450 milliliters per minute. Prior to insertion in the cell the anode is repeatedly washed, dried and weighed until weights agreeing within 0.1 milligram are obtained. Operation is then commenced and the anodes are removed when desired, usually overy 100 operating hours, and reweighed to the same criteria, the difference representing the wear of the anode.

Example 1 Coating solutions are prepared by dissolving tetrabutyl orthotitanate, RuCl -2.5H O and CoCl -6H O in normal butyl alcohol containing about 6 percent by volume of 36 percent HCl. The amounts of the titanium, ruthenium and cobalt salts used are those sufiicient to give the mole ratios indicated in Table l, i.e., for Sample E, 317, 63 and 47 grams per liter, respectively, of the titanium, co-

bait and ruthenium salts are used. Six coats of each solution are applied to clean titanium mesh substrates with heating after each application in air to a temperature of 450 C. The temperature of the substrate is brought rapidly (one minute or less) to temperature, which is maintained for seven minutes. After application of the last coating, samples B-F are post-baked for 7 minutes at 550 C. Sample A is not so treated since it v is known that heating at elevated temperatures has an adverse effect upon the potential of standard solid solution-type coatings. 0n the other hand, in Sample G thermochemical decomposition is effected at 300 C. and the post-bake temperature is 600 C. The values given for chlorine potential are obtained by measuring the half-cell potential (vs. normal hydrogen electrode) at which chlorine is discharged at the anodic surface at a current density of 6 a.s.i. and a temperature of C.

From the foregoing table it will readily be seen that up to 60% of the ruthenium may be substituted with cobalt without any detrimental effect on the electrode potential. X-ray analysis indicates that greater than 50% of the cobalt is converted to crystalline cobalt metatitamate in all samples except A and G, A being the solid solution and G, evidencing no diffraction pattern, being amorphous cobalt oxides. No titanate patterns, other than cobalt metatitanate, are present in any of the samples.

Example 2 This example illustrates the importance of temperature control in the thermochemical deposition method of the present invention. Anodes are prepared as in Sample E of Example 1 with the exception that (a) Sample E1 is merely baked between coats for seven minutes at 450 C.; (b) Sample E2 is baked for seven minutes between coats at 450 C. and post-baked for seven minutes at 550 C. and (c) Sample E3 is baked for seven minutes between coats at 550 C. X-ray analysis shows the following coating characteristics (El) rutile solid solution (TiO -RuO no crystalline cobalt compounds; (E2) rutile solid solution, crystalline cobalt metatitanate, (E3) rutile solid solution, crystalline cobalt metatitanate. The chlorine potential of sample E3, measured at 6 a.s.i., is 0.06 volt higher than sample E2. Furthermore, a solubility test conducted by boiling the samples for 10 minutes in 10 volume percent HCl indicates that 0.40 milligram of ruthenium and 4.5 milligrams of cobalt are leached from sample E1 whereas only 0.13 and 3.2 milligrams, respectively, are leached from E2.

Example 3 In this example, certain of the samples from Examples 1 and 2 are subjected to the wear-rate test previously described with the results shown in Table II.

l N o post-bake.

Two conclusions are reached. The crystalline cobalt metatitanate-modified solid solution coatings are superior to the unmodified coatings of the prior art insofar as wear-rate is concerned, often without a sacrifice in voltage. Further, while its voltage is still favorable, a cobalt-modified coating, absent the post-bake treatment and hence non-crystalline, exhibits a high wear-rate (E1).

Although the invention has been described with reference to certain preferred embodiments thereof, it is not to be so limited since changes and alterations may be made therein which are still within the full and intended scope of the appended claims.

We claim:

1. An article having a coating thereon, which coating consists essentially of a coherent mixture consisting essentially of crystalline cobalt meta-titanate and a valve metal oxide-platinum metal oxide solid solution.

2. A coating as in claim 1 wherein the mole ratio of valve metal to platinum metal plus cobalt is within the range of 5:1;1:5 and the ratio of platinum metal to cobalt is within the range of 4:1-2:3.

3. A coating as in claim 1 wherein the valve metal: platinum metalzcobalt mole ratio is about 10:2:3.

4. A coating as in claim 1 wherein the valve metal is titanium and the platinum metal is ruthenium.

5. A coating as in claim 1 wherein the amount of crystalline cobalt metatitanate is within the range of 10- 35% by weight of the total crystalline cobalt metatitanate plus rutile solid solution,

6. An electrode which consists essentially of a supporting substrate bearing on at least a portion of the surface thereof Ian electrically-conductive and electrooatalytically-active coating which comprises a coherent mixture consisting essentially of crystalline cobalt metatitanate and a valve metal oxide-platinum metal oxide solid solution.

7. An electrode as in claim 6 wherein the substrate is an electrically-conductive substrate.

8. An electrode as in claim 6 wherein the substrate is a valve metal substrate.

9. An electrode as in claim 6 wherein the valve metal and the platimum metal in the solid solution are, respectively, titanium and ruthenium.

10. An electrode as in claim 6 wherein the coating Q has a valve metal to platinum metal plus cobalt mole ratio of 5:1-1 :5 and a platinum metal to cobalt ratio of 4:1-223.

11. An electrode as in claim 6 wherein the valve metalzplatinum metalzcobalt mole ratio in the coating is about 10:2:3.

12. An electrode as in claim 6 wherein the coating is dispersed in a resin matrix applied to the supporting substrate.

13. An electrode as in claim 12 wherein the resin is a fluorocarbon resin.

-14. An electrode as in claim 6 wherein the coherent mixture is applied to a substrate bearing an oxygen barrier layer coating.

15. A coating as in claim 6 wherein the amount of crystalline cobalt metatitanate is within the range of 10- 35% by weight of the total crystalline cobalt metatitanate plus rutile solid solution.

16. An electrode which consists essentially of a valve metal substrate bearing on at least a portion of the surface thereof a coating comprising a coherent mixture consisting essentially of crystalline cobalt metatitanate and a titanium dioxide-ruthenium dioxide solid solution.

References Cited UNITED STATES PATENTS 3,505,118 2/1970 'Mehra et a1. 136-86 3,632,498 1/ 1972 Beer 204-290 F 3,684,543 8/1972 de Nora et al. 204-290 F 3,645,862 2/ 1972 Cotton et a1. 204290 F OTHER REFERENCES The Condensed Chemical Dictionary, 7th ed., 1967, Reinhold Book Corp., N.Y., pp. 239, 696, 948.

FREDERICK EDMUNDSON, Primary Examiner US. Cl. X.R. 136-86, PC 

